Limiting Factors for Stable Operating Conditions of 1G and 2G High-T Superconducting Current-Carrying Elements in High Magnetic Fields

ABSTRACT

The limiting currents stably flowing in the current-carrying elements based on 1G and 2G high-temperature superconductors (HTS) are studied. An analysis of the stability of macroscopic thermal and electrical states in the current charging modes is made in terms of the static zero-dimensional and one-dimensional models. For the coated tapes based on the YBa₂Cu₃0₇ (Y123), the stability investigation was made varying the thicknesses of the copper or silver, external magnetic field and cooling conditions. The analysis performed reveals that the currents of the instability do not practically depend on the thicknesses of copper or silver when the superconducting current-carrying element made of the Y123 is cooled by the cryocooler. At the same time, the increase in the thickness of the stabilizing copper layer leads to a monotonic increase in the limiting stable currents, if the liquid helium (LHe) is used as a coolant. The mechanisms of these features are discussed. It is shown that the limiting stable currents and the induced electric fields depending on the cooling conditions can be both below (subcritical regime) and above (overcritical regime) of a priori defined critical values of the electric field and current of superconductor. The stability criteria of DC regimes are written for the superconducting wires, which structurally consist of superconducting multi-filaments placed into a non-superconducting matrix. They permit one to estimate the allowable limiting values of the charged current, the induced electric field and the overheating of superconductor, which can lead to both the subcritical and overcritical stable regimes. The boundary conditions of them are formulated. In particular, the peculiarities of the stable limiting current regimes of Ag-sheathed Bi₂Sr₂CaCu₂O₈ (Bi2212/Ag) wire are discussed in detail. As a result, the investigation performed extends significantly the class of admissible states of HTS as it proves the existence of stable resistive modes in which they do not go to the normal states in spite of the overcritical operating states that take place under different cooling conditions.

Keywords:

High-temperature superconductors, voltage-current characteristic, current instability, overheating, flux creep.